Thermoplastic resin compositions improved in rigidity by blending rubber-reinforced aromatic vinyl resins represented by ABS resins and AES resins with fillers such as glass fiber are broadly used as molding materials for molded articles requiring mechanical strengths such as impact resistance and rigidity, for electric/electronic devices, office automation devices, household appliances, vehicle components, sanitary supplies and the like.
However, when molded articles to be directly touched by hand, such as assist grips of automobiles, are molded from thermoplastic resin compositions comprising fillers as described above, the following problems occur, though molded articles excellent in rigidity can be obtained: the surface of the molded articles feels rough when the molded articles are touched by hand; and squeaking noise (rubbing noise) is often generated when a pair of the molded articles, or the molded article and another molded article composed of another resin such as polyethylene, polyester and polyvinyl chloride, are used so as to be contacted with and rubbed against each other. The squeaking noise is known as an abnormal sound caused by the stick-slip phenomenon generated when two objects are rubbed against each other, and is a property different from slidability of resins.
The stick-slip phenomenon is understood as a phenomenon in which frictional force largely varies periodically as illustrated in FIG. 13; and more specifically, it is generated as illustrated in FIG. 14. That is, in the case where an object M connected to a spring is placed on a driving board moving at a driving velocity V as shown in the model of FIG. 14(a), the object M first moves to the right direction together with the board moving at the driving velocity V by the action of a static frictional force as illustrated in FIG. 14(b). Then, when the force of the spring exerted so as to restore the object M becomes equal to the static frictional force, the object M starts to slip in the direction opposite to the driving velocity V. At this time, since the object M receives a dynamic frictional force, the slipping stops at the time point of FIG. 14(c) when the force of the spring becomes equal to the dynamic frictional force, that is, the object M sticks to the driving board, and results in again moving in the same direction as the driving velocity V (FIG. 14(d)). This is called the stick-slip phenomenon; and it is said that, as illustrated in FIG. 13, if the difference Δμ between a static friction coefficient μs and a friction coefficient μl which is the lower end of the sawtooth wave is large, the squeaking noise becomes liable to occur. Here, the dynamic friction coefficient is a value midway between the μs and the μl. Therefore, even when the absolute value of the static friction coefficient is low, the squeaking noise becomes liable to occur, if the Δμ is large. Such squeaking noise becomes a main cause for impairing comfortableness and quietness in automobile cabins, offices and house rooms, and reduction of squeaking noise is strongly demanded.
Although a thermoplastic resin composition comprising a rubber-reinforced aromatic vinyl resin blended with a crystalline resin, polybutylene terephthalate (PBT) is conventionally known, it just aims at improvement of coatability (Patent Document 1 and Patent Document 2); no disclosure is made about the system in which fillers such as glass fiber are incorporated; or no disclosure or suggestion is made about the improvement of hand touch feeling or the squeaking noise of the resin composition comprising glass fiber.